The present invention relates to a resonating knock sensor that is secured to a suitable place within an internal combustion engine such as, for example, the wall of the cylinder, for the purpose of detecting the occurrence of knocking within the cylinder.
A knock sensor of the type contemplated by the present invention consists of a vibration sensing unit that is supported within a casing to be installed on an engine and which comprises a piezoelectric element attached as an integral part to one surface of a thin resonating metal sheet. The vibration sensing unit has a resonance frequency equal to the frequency of the oscillating wave from knocking, so that when knocking occurs, the piezoelectric element will vibrate in an effective manner to produce a maximum output in accordance with this vibration.
Unlike the non-resonating type that senses knocking by imposing a compressive strain on a fixed piezoelectric element in synchronism with the vibration from knocking, the resonating knock sensor does not produce oscillations in synchronism with the vibration of the thin metal sheet and, instead, the profile of the vibratory output obtained has slow rise and fall times in that the amplitude of the oscillating wave gradually increases after the occurrence of knocking and gradually decreases after completion of the knocking. The electronic circuit for this sensor has a certain threshold and any oscillations that occur within the rise and fall times and which are below the threshold value are processed as noise components. Therefore, if knocking that has a waveform containing a few number of peaks occurs, no output signal can be detected from the sensor since the vibration will disappear before it reaches the threshold level.
On the other hand, if the engine is running at an increased rotational speed and knocking occurs at very short intervals, the falling component of the vibration resulting from one knock will be superposed on the rising component of the vibration from the next knock, and the amplitude of the overall vibration is increased to produce a higher noise level. In order to ensure accurate detection of such knocking-associated vibration, the threshold value for distinguishing any noise from the desired signal is determined by averaging the noise components, but then the threshold level is increased to degrade the S/N ratio of the sensor. A clearer picture of this phenomenon will be obtained from FIG. 4; if knocking occurs at fairly long intervals, the threshold value T will be proper in connection with the first signal S1, but if the interval of knocking decreases, the valley V of the signal will be elevated because of the superposition of the falling component of the first wave on the rising component of the subsequent wave. In order to avoid any disadvantages that may result from this superposition, the circuit for the sensor is so designed that the threshold level is determined by averaging the noise components below the value T, but then this provides a higher value of T. Eventually, the difference between the peak output of a signal S4 and the threshold level T becomes so small as to cause a progressive decrease in the S/N ratio. In other words, the higher the rotational speed of the engine and the shorter the interval of the occurrence of knocking, the more degraded the S/N ratio is.
In spite of these serious disadvantages, the resonating knock sensor is extensively used because it does not have to use an electrical filter, which is indispensable to the non-resonating type, and enables simplified signal processing.